It is known to use set accelerating chemical admixtures to improve early strength in cementitious mixtures such as shotcrete. For example, depending on the mix design, set accelerators have achieved a two MegaPascal (MPa) of compressive strength in shotcrete mixtures within three hours after application.
Due to fast set acceleration, such chemical admixtures achieve desired early strength; but, unfortunately, they also bring about a number of disadvantages. These disadvantages include rebound waste of the shotcrete, consolidation and compaction issues, as well as long-term strength concerns and decreased plasticity (e.g., zero slump) of the shotcrete mixture.
The industry has been using silica fume or colloidal silica, due to a pozzolanic effect on early strength enhancement of cement binder, or, as another example, due to reduced rebound in shotcrete applications. However, the high surface area of silica increases water demand in cementitious mixes, reduces workability, and increases viscosity, all of which adversely affect pumpability of cementitious mixes.
The construction industry actively seeks a solution that can not only improve the early-age strength development and rebound reduction achieved with colloidal silica, but also that can offset the impact of any set accelerator, which is otherwise detrimental to the performance of the cementitious mixes. Applicants believe that industry demand for even faster strength development should be addressed. For example, the industry is using shotcrete increasingly for ground support purposes, in temporary as well as permanent excavation structures, and for mining and tunneling applications. Moreover, architects and engineers appear to be relying increasingly upon shotcrete to carry immense loads and to provide structural integrity at faster speeds. Thus, early-age strength enhancement is desirable for achieving safety as well as productivity goals. The present inventors believe that even the smallest improvements in early-age strength enhancement can help to obtain significant benefits in all of these areas, particularly in time-sensitive applications where strength gain rate is crucial to ensure structural integrity in underground environment for safety.
There continues to be a need for supplementary additives to enhance early-age strength and also to offset the detrimental impact of accelerators on rebound in shotcrete applications, as well as to achieve compaction properties in hydratable cementitious systems as a whole.